How do you make a mitred corner?
I would suggest using the shear tool to do this.

With the vertices at one end selected go into the side view (shear deforms based on the view) &#9096; Ctrl&#9095; Alt&#8679; ShiftS and then enter a value of 1 (or -1) to get a 45 degree angle. If your pivot point is set to median then you may need to move the edge sideways to re-align it or you can position the 3D cursor and set the pivot point before shearing.